Tomographic Measurements of Carbon Monoxide Temperature and Concentration in a Bunsen Flame Using Diode Laser Absorption

The temperature and concentration profiles of carbon monoxide (CO) in a laminar, axisymmetric, premixed methane-air Bunsen flame are measured using line-of-sight diode laser absorption spectroscopy in conjunction with computer tomographic (CT) reconstruction. The absorption spectra for the P(20) (v = 2 1) and P(27) (v = 1 0) vibrotational transitions of CO were measured at 21 evenly spaced positions over a 1.33 cm span for a 1.3 cm radius flame. The CT reconstruction algorithm was based on the Fourier convolution technique. The tomographically reconstructed normalized transmission profiles derived from the absorption spectra, in conjunction with a quantum mechanical model for the vibrotational behavior of CO, yielded both the temperature and concentration profiles. The Bunsen flame investigated had 3 distinct zones: (1) an inner rich-premixed flame zone, (2) an outer non-premixed flame zone and (3) an unburnt core region. The reconstructed temperature profile showed that the core region temperature was close to ambient and rapidly increased to the adiabatic flame temperature at the inner flame zone. The temperature was observed to reach a local minima of approximately 200 K less than the adiabatic flame temperature. At the outer flame zone, the temperature increased to the adiabatic flame temperature at the outer flame zone. The reconstructed concentration profile was reasonable, low in the core region, with a maximum at the approximate location of the inner fuel-rich flame zone and subsequently decreased to a low level towards the outer flame zone where the CO concentration goes towards zero as expected for a non-premixed flame zone. An advantage of the tomographic technique is that the knowledge of the laser pathlength across the actual flame is not required for quantitative measurements of the concentration. In an attempt to measure the low concentrations of CO as it occurs at the edge of a Bunsen flame, we switched from direct absorption to the second harmonic technique. With this improved sensitivity, we have discovered a large spectral shift of the CO P(38) (v = 1 0) transition depending on whether the post flame combustion products are fuel-lean (blue shift +0.009 cm−1) or fuel-rich (no shift).